A 465 ha area in northwestern New Jersey was divided into 3 blocks for treatment. Of this area, 183 ha were treated with Sevin 4 oil (carbaryl), 176 ha were treated with Dipel (Bacillus thuringienisis) and 106 ha were left untreated. Trees in the area were mostly chestnut oak and white oak. The host population of gypsy moth ranged from 1255 to 3027 egg masses/0.4 ha. Dipel was applied twice, each application at a dosage of 8 BIU/0.4 ha. The rate of application was 7.6 l/0.4 ha. Sevin 4 oil was applied once at a dose of 0.45 kg AI/0.4 ha; the rate of application was 0.95 l/0.4 ha. Both chemicals were applied for the first time on May 30, at which time at least 75% of the larvae were in the 2nd and 3rd stages, and white oak foliage had expanded by 50%. Dipel was also applied on June 6. Adult, gypsy moth-parasite insects were collected in 1973 and 1974 from the 3 areas at pre- and post-spray intervals. No significant differences in the numbers of any insect species were observed among the areas before spraying. Significantly fewer Brachymeria intermedia and Sarcophagidae, Braconidae, Chalcididae, Sarcophagidae and common species of Tachinidae were collected from treated blocks. Reasons for the reduced numbers of parasites include direct mortality from the insecticides and indirect mortality from host population reduction.